The present invention relates to devices and methods for contactlessly drying and guiding traveling webs, and more particularly, an improved air flotation turning device and method particularly suitable for guiding the webs in an arcuate path.
In web printing and drying operations, it is often desirable that the web be contactlessly supported in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web. Occasionally it becomes necessary to change the direction of web travel while maintaining the contactless environment. This can be accomplished using air turns, which are devices that support a flexible web on a cushion of air pressure as the web travels around a curved path. Air turns have a generally partially cylindrical surface through which pressurized air is introduced through various slots, holes or apertures, or other designs or patterns. Typical air turns which are commercially available are a 95xc2x0 turn, which carries the web around a 95xc2x0 arc, and a 20xc2x0 xe2x80x9cshallow wrapxe2x80x9d turn, which carries the web around an arc of 20xc2x0.
Such air turns replaced grater rollers. Grater rollers were a means to turn the web utilizing frictional contact with the web. As a result, web marking problems often arose. Although the use of air turns eliminated marking problems, the absence of the additional frictional restraint provided by the rollers led to web tracking problems, especially in the case of xe2x80x9cbaggyxe2x80x9d or non-uniform webs. To compensate for tracking problems, the air turn is used as a steering device. By tilting one edge of the air turn in a direction perpendicular to and toward the web, a force is provided tending to push the web away from that side. Conversely, if that end of the air turn were moved away from the web, the resulting air pressure forces pulls the web toward that end. Optical sensors are used to monitor web drift and send a signal to the steering drive motor controlling the position of the air turn. The drive motor moves the operator end of the air turn. Alternatively or additionally, the air turn could be tilted manually.
One example of an air turn is that disclosed in U.S. Pat. No. 4,182,472 (the disclosure of which is herein incorporated by reference). Specifically, a guide for contactless support of a running web as the latter changes directions is provided. The guide is formed as a drum-like member having an arcuately curved surface which can be variable as to the length of its arc, depending on the degree of turn or change of direction desired for the running web. A series of parallel grooves extending in the direction of web travel are formed in the arcuate surface of the drum-like member. An air nozzle extends along the length of the drum-like member and at each end of the groves, and pressurized air is fed through the nozzles so as to form a pneumatic cushion between the web and the arcuate surface and thereby float the web. The grooves in the arcuate surface act as labyrinth seals in inhibiting the transverse air flow out of the cushion and towards the edges of the running web.
A further example of an air turn is provided in U.S. Pat. No. 2,689,196, wherein a series of holes are formed in the cylindrical surface for the passage of pressurized air therethrough to support and guide a web passing over the drum. Similarly, U.S. Pat. No. 3,097,971 discloses a device having a series of slits in the curved supporting surface and which extend longitudinally and/or transversely to the web. Air under pressure is passed through these slits to form a cushion between the drum and the web.
An important aspect of any flotation system is the stability of the web as it passes over the air bar. Airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the drying, resulting in coating or web damage. Web flutter can be manifested in a multitude of forms, ranging from a violent flapping of the web to a high frequency drumming.
Excessive web flutter has been encountered in conventional air turn applications. Where a plurality of air turns are used together so that the web follows a sinusoidal path, web flutter has been encountered as the web leaves the lower air turn and before it reaches the upper air turn.
It is therefore an object of the present invention to minimize or eliminate web flutter during the contactless support of a floating web.
It is a further object of the present invention to provide a flotation device with increased cushion pressure to properly float heavy or curled webs around the device.
It is still a further object of the present invention to provide a flotation device that results in uniform flotation height about the device even at different or fluctuating web tensions.
The problems of the prior art have been overcome by the present invention, which provides an air turn for supporting and optionally drying a web, the air turn comprising an arcuate surface having a plurality of apertures formed therein. Pressurized air is supplied to the air turn cavity, and exits through the plurality of apertures to form a cushion of air to float the web. Turn angles of about 90xc2x0 to about 300xc2x0 with no contact to the web are achievable with the apparatus of the present invention. The unique design allows for non-contact flotation and turning of a tensioned web with low pressure requirements, reduced air spillage and reduced noise level.
The shape of the apparatus allows for void or open areas inside the web entering and exiting the air turn. These void or open areas are critical in maintaining a stable web flotation entering and exiting the turn. The void areas created by the shape of the turn and the special size and spacing of the pressure slots make the turn superior in operation to conventional non-contact turning devices. Conventional turning devices can require as much as 30-50% higher pressure requirements that the device of the present invention.